A resistance temperature detector (RTD) is a sensor used to measure temperature by correlating the resistance of the RTD element with temperature. The RTD element may consist of a length of fine coiled wire wrapped around a ceramic or glass core. The RTD element may be made from a higher thermally conductive material such as platinum, nickel, or copper. The material has a predictable change in resistance as the temperature changes, which is used to determine temperature. Further, the RTD may be used for measuring the temperature of solids, liquids and gases due to its long-term stability, ease of recalibration, repeatability, and precision over narrow temperature ranges. The RTD element may be fragile, so it is often placed inside a probe to protect it. However, the probe reduces the RTD's response time due to heat having to first transfer through the cover before reaching the RTD.
In one definition, thermal conductivity is the property of a material to conduct heat. Heat transfer across materials of higher thermal conductivity occurs at a higher rate than across materials of lower thermal conductivity. Further, materials of higher thermal conductivity are widely used in heat sink applications and materials of lower thermal conductivity are used as thermal insulation. A higher thermally conductive material such as copper has a thermal conductivity ranging from about three hundred and eighty-five Watts per meter Kelvin (385 Wm−1K−1) to about four hundred and one Watts per meter Kelvin (401 Wm−1K−1) at two hundred and ninety-three degrees Kelvin (293° K). A lower thermally conductive material such as glass has a thermal conductivity ranging from about eight-tenths Watts per meter Kelvin (0.8 Wm−1K−1) to about one and four-tenths Watts per meter Kelvin (1.4 Wm−1K−1) at two hundred and ninety-three degrees Kelvin (293° K).
In one example, the RTD may be made by disposing a conductive metal film onto a semiconductor substrate and etching the conductive metal film into a resistance temperature detector pattern. Inexpensive metals such as copper or nickel may sometimes be used as the conductive metal. However, these metals may be restricted in their temperature measurement range due to, for instance, their non-linear temperature-resistance relationships. To avoid this limitation, RTD designs may use platinum as the conductive metal. Due to its chemical inertness, platinum has a nearly linear temperature-resistance relationship to enable an RTD to more precisely measure temperature. However, to remain stable, platinum should remain in its pure form and be shielded from high temperatures or harsh environments. A variation of this cover for an RTD has been used to shield platinum from high temperatures or external environments. However, this variation may share some of the aforementioned limitations such as having a slow response time due to heat transfer. Thus, probes may not typically be used in applications requiring a rapid-response time. This may be especially true in applications requiring high temperatures or harsh environments, such as those commonly found in automobile engines and jet engines. In addition, the measurement of fluid temperature is important in many applications. In one definition, a fluid is a liquid or a gas. When the fluid is a liquid this measurement is easier to make. The higher density and heat capacity may mean that a larger probe with a higher thermal mass may be used without affecting the response time or accuracy adversely. However, when the fluid is a gas, the task may be more difficult. To measure air or other gases, an exposed sensor such as a thin film RTD or thermocouple may be used. These sensors may not be very robust and their use outside of a laboratory setting may be limited.
In another example, FIG. 1 illustrates a partial longitudinal cross-sectional view of a prior art temperature probe structure 100. The temperature probe structure 100 is configured to include a longitudinal probe 101, an RTD 103, a header 105, and a housing 107. Further, the header 105 and the housing 107 are disposed around and form a cavity 109. The probe structure 100 is disposed about a central axis 120, as shown in FIG. 1. The longitudinal probe 101 has a front portion and a back portion. The front portion of the longitudinal probe 101 is exposed such as to an external environment to be measured. The back portion of the longitudinal probe 101 is disposed within the cavity 109 of the header 105. The longitudinal probe 101 is coupled to the housing 107 using the header 105 as a thermally conductive seal. The header 105 and the housing 107 is made of a thermally conductive material such as metal. The longitudinal probe 101 is also made of a thermally conductive material such as metal. In one instance, the longitudinal probe 101 is welded to the header 105. In another instance, the longitudinal probe 101 is brazed to the header 105 such as by soldering with an alloy of copper and zinc at high temperature.
In FIG. 1, the RTD 103 is placed within the longitudinal probe 101. The metal-to-metal contact between the longitudinal probe 101 and the header 105 increases the likelihood of heat dissipating throughout the temperature probe structure 100. Thus, the temperature probe structure 100 is more conducive to measuring temperatures of a liquid since the thermal mass of the longitudinal probe 101 is smaller compared with the thermal mass of the liquid. However, the temperature probe structure 100 is less conducive to measuring temperatures of a gas since the thermal mass of the longitudinal probe 101 is too high for an accurate or fast measurement. Accordingly, there is a need to improve the measurement response and accuracy of fluid temperatures.